The coiled tubing of the present invention is made out of a suitable metal such as high strength low alloy carbon steel and is primarily intended to be used in the oil and gas well servicing industry. For this particular application, the tubing typically has a diameter between 1 to 5 inches and a length between 12,000 to 20,000 feet, and is wound onto a reel or spool, the diameter of which must be restricted due to transportation requirements. For this reason the plastic limits of the tubing are exceeded during the initial coiling process, and when the tubing is deployed into a well bore to different depths. Also, in many cases, upon completion of the required work, the tubing is rewound onto the spool and moved to another well for reuse. During such coiling and uncoiling, the tubing is subjected to low cycle fatigue which produces cracks that ultimately cause the tubing to fail. Also, stress corrosion cracking of the tubing commonly occurs when the tubing is exposed to hydrogen sulfide in oil and gas wells.
Due to the nature of the coiling and uncoiling operations and the strain limits of the tubing material, low cycle fatigue is unavoidable. However, to the extent that the development and growth of fatigue cracks can be retarded, the working life of the coiled tubing string will be proportionately extended. Likewise, the working life of the tubing will be extended if the stress corrosion cracking of the tubing is reduced.